a) Field of the Invention
The invention is directed to an apparatus for the generation of soft x-radiation, particularly EUV radiation, by means of a laser-induced plasma. The invention is primarily applied for spatially stable generation of radiation in exposure machines for semiconductor lithography.
b) Related Considerations and Art
Dense, hot plasma that is generated on the basis of a gas discharge or laser excitation can emit EUV radiation under defined excitation conditions. In the latter case, a so-called target is heated by a laser to the point that a significant proportion of the characteristic and temperature radiation coming from it lies in the extreme ultraviolet (EUV) spectral region. The target with a suitable emission characteristic is a stationary material or a continuous or discontinuous flow of material of solid, liquid, gaseous or mixed consistency. These different forms will be referred to hereinafter generally as target.
In practice, fluctuations in the radiating direction and radiating angle of the laser beam result in temporal fluctuations in the EUV emission of the plasma because, when the laser beam varies in direction and radiating angle, these variations are transmitted to the point of interaction with the target and the intensity distribution at this point. However, for many applications, particularly for a radiation source for projection lithography, the emitted radiation output may undergo only small to very small temporal fluctuations in the EUV spectral region.
As a rule, the target must interact with a focused laser beam in order to achieve the most efficient possible generation of short-wavelength electromagnetic radiation in the desired wavelength region. For this purpose, the laser beam generated by the laser system is directed to the target in a focused manner by means of a beam-shaping element such as a lens or a concave mirror.
Fluctuation and drifting of the beam direction and/or radiating angle in laser beams, particularly those coming from a high-power laser, regularly occur primarily as a result of thermal changes. These fluctuations progress into variations in position and in power density at the point of interaction with the target and lead to unwanted fluctuations in the emission of EUV radiation particularly in the case of a target jet.
In order to meet the strict requirements for stability with respect to the position and the output of a radiation source for EUV projection lithography, the interaction point and the interaction surface between the target and laser beam must be maintained stable with respect in time. A known possibility for stabilizing the beam direction of a laser consists in measuring and regulating the beam direction, e.g., by means of a CCD camera as sensor and a micromirror or tilting mirror as actuating member.
A possibility for stabilizing the position of a target flow, e.g., a filament target, jet target or droplet target, is described in Patent Application DE 103 14 849 which has not yet been published. This patent application discloses an arrangement for stabilization having optical sensors for detecting deviations in at least one radiating direction of the target jet or energy beam from an intersection of the two radiating directions that is provided as interaction point. The output signals of the optical sensors are used to align the radiating directions with the interaction point by adjusting and tracking at least one of the radiating directions of the target jet and energy beam in the manner of a control loop. However, regulation of this type is disadvantageous on the whole because it is relatively complicated, requires active adjusting members, and is limited with respect to the speed of regulation.